Connective tissues comprise a diverse group of tissues, including tendon, cartilage, bone, skin, and vitreous which require specific mechanical and structural properties appropriate to their function. These tissues all have collagen fibrils as a major structure component, and their required tissue specific properties depend on the molecular and supramolecular organization of collagen together with other molecules in the extracellular matrix. Our laboratory has embarked on a program to characterize the structural differences in a range of connective tissues with different genetic types and different non-collagenous components, at the level of molecular and fibril organization. Tissue specific variations in collagen structure will be related to the interactions which stabilize these aggregates. The molecular packing and molecular interactions will be characterized for D-periodic fibrils formed by different genetic types of collagens. Fibrils of types I, II and III collagens differ in molecular density, and the mediation of such density differences by water interactions, sequence differences and carbohydrate content will be examined. The minor collagens type V and 1 alpha, 2 alpha, 3 alpha can form fibrils which appear to be of very uniform small diameter and to have an axial D period. The fibril structures, fibril forming process, and structural effects of terminal peptides found in intact tissue forms will be characterized for these minor collagens. The organization of fibrils in tissues and the effects of proteoglycan on collagen organization will be examined for type II collagen in notochord. The small uniform type II fibrils in lamprey notochord have a large center to center distance, equal to almost twice the fibril diameter. The forces stabilizing this array of fibrils will be investigated, to determine if they involve an equilibrium state, e.g. repulsion of surface charges on fibrils, or more rigid structures, e.g. covalent cross-linking or a coating of the fibril surface. The role of proteoglycan in collagen structure will be examined by determining the effect of proteoglycan removal on collagen molecular packing, fibril diameter, and interfibrillar distance, and on the charge and water interactions between collagen fibrils.